Neutralization system for electrochemical chlorine dioxide generators

ABSTRACT

An electrochemical chlorine dioxide generator with a neutralization system includes a pH treatment tank for receiving waste products from the electrochemical generation process such as anolyte and catholyte wastes. A neutralization solution tank is fluidly connected to the pH treatment tank. A neutralization feed pump transfers a neutralization solution to the pH treatment tank. A recirculation pump for mixing the waste products is fluidly connected to the pH treatment tank. An effluent holding tank is fluidly connected to the pH treatment tank. A transfer pump transfers the neutralized waste products from the pH treatment tank to the effluent holding tank.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part of U.S. patent application Ser. No. 10/902,681 filed on Jul. 29, 2004, entitled “Chlorine Dioxide Solution Generator.” This application is also related to U.S. patent application Ser. No. 11/145,398 filed on Jun. 3, 2005, entitled “Chlorine Dioxide Solution Generator With Temperature Control Capability”, which is, in turn, a continuation-in-part of the '681 application. This application is also related to U.S. patent application Ser. No. 11/289,813 filed on Nov. 30, 2005, entitled “High-Capacity Chlorine Dioxide Generator”, which is also a continuation-in-part of the '398 application. The '681, '398 and '813 applications are each hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to chlorine dioxide generators and to the use of such generators in water treatment systems. More particularly, the present invention relates to an electrochemical chlorine dioxide generator, with a neutralization system for waste products from chlorine dioxide production.

BACKGROUND OF THE INVENTION

Chlorine dioxide (ClO₂) has many industrial and municipal uses. When produced and handled properly, ClO₂ is an effective and powerful biocide, disinfectant and oxidizer.

ClO₂ is also used extensively in the pulp and paper industry as a bleaching agent, but is gaining further support in such areas as disinfection in municipal water treatment. Other end-uses can include as a disinfectant in the food and beverage industries, wastewater treatment, industrial water treatment, cleaning and disinfections of medical wastes, textile bleaching, odor control for the rendering industry, circuit board cleansing in the electronics industry and uses in the oil and gas industries.

In water treatment applications, ClO₂ is primarily used as a disinfectant for surface waters with odor and taste problems. It is an effective biocide at low concentrations and over a wide pH range. ClO₂ is desirable because when it reacts with an organism in water, chlorite results, which studies to date have shown does not pose a significant adverse risk to human health at chlorite concentrations less than 0.8 parts per million (ppm). The use of chlorine, on the other hand, can result in the creation of chlorinated organic compounds when treating water. Such chlorinated organic compounds are suspected to increase cancer risk.

Producing ClO₂ gas for use in a ClO₂ water treatment process is desirable because there is greater assurance of ClO₂ purity when ClO₂ is in the gas phase. ClO₂ is, however, unstable in the gas phase and will readily undergo decomposition into chlorine gas (Cl₂), oxygen gas (O₂) and heat. The high reactivity of ClO₂ generally requires that it be produced and used at the same location. ClO₂ is, however, soluble and stable in an aqueous solution.

The production of ClO₂ can be accomplished both by electrochemical and reactor-based chemical methods. Electrochemical methods have an advantage of relatively safer operation compared to reactor-based chemical methods. In this regard, electrochemical methods employ single precursor, namely, a chlorite solution, unlike the multiple precursors employed in reactor-based chemical methods. Moreover, in reactor-based chemical methods, the use of concentrated acids and chlorine gas can pose safety concerns.

Electrochemical cells are capable of carrying out a selective oxidation reaction of chlorite to ClO₂. The selective oxidation reaction product is a solution containing ClO₂. To further purify a ClO₂ gas stream, the gas stream is typically separated from the solution using a stripper column. In the stripper column, air is passed from the bottom of the column to the top while the ClO₂ solution travels from top to the bottom. Pure ClO₂ is exchanged from solution to the air. Suction of air is usually accomplished using an eductor, as described in copending and co-owned application Ser. No. 10/902,681, of which the present application is a continuation-in-part.

As described in the '681 application, ClO₂ can be prepared a number of ways, generally via a reaction involving either chlorite (ClO₂ ⁻) or chlorate (ClO₃ ⁻) solutions. The ClO₂ created through such a reaction is often refined to generate ClO₂ gas for use in the water treatment process. The ClO₂ gas is then educed into the water selected for treatment. Eduction occurs where the ClO₂ gas, in combination with air, is mixed with the water selected for treatment.

While electrochemical generators are suited for generating chlorine dioxide, such generators produce undesirable waste products. These waste products are produced both in the anolyte and catholyte loops of the electrochemical generator. Anolyte waste products typically result from residual reactants, side reaction products, and chlorine dioxide that remains unstripped from the stripper column. Other anolyte impurities can include sodium chlorite, sodium chlorate, sodium chloride, sodium sulfate, hypochlorous acid and chlorine dioxide. The operation of the catholyte loop in an electrochemical chlorine dioxide generator also produces caustic soda (that is, sodium hydroxide). Although sodium hydroxide can be used in other reactions, when catholyte production is low it can be more economical to dispose of it. The present apparatus and method effectively neutralize waste products from the anolyte and catholyte loops of electrochemical chlorine dioxide generators.

SUMMARY OF THE INVENTION

An electrochemical chlorine dioxide generator comprises an anolyte loop fluidly connected to a neutralization system. A catholyte loop is fluidly connected the neutralization system and the neutralization system treats (that is, neutralizes) waste products from the anolyte loop and/or the catholyte loop. In a preferred embodiment, an absorption loop is fluidly connected to the anolyte loop.

In an embodiment, the electrochemical chlorine dioxide generator has a stripper column within the anolyte loop that is fluidly connected to the neutralization system. In a further embodiment, a probe monitors the pH of the waste products in the neutralization system. In another embodiment, at least one of the electrochemical chlorine dioxide generator and the neutralization system is controlled by a programmable logic controller (PLC).

In a preferred embodiment, the neutralization system comprises a pH treatment tank for receiving waste products that is fluidly connected to at least one of the anolyte loop and the catholyte loop. The neutralization system can further comprise a neutralization solution tank fluidly connected to the pH treatment tank. In a further embodiment, a recirculation pump is fluidly connected to the pH treatment tank, and the recirculation pump mixes the waste products in the pH treatment tank.

In a preferred embodiment, the electrochemical chlorine dioxide generator further comprises an effluent holding tank fluidly connected to the pH treatment tank. The effluent holding tank receives neutralized waste products. The electrochemical chlorine dioxide generator can further comprise a transfer pump for transferring the neutralized waste products to the effluent holding tank. A diverting valve controls the fluid connection between the pH treatment tank and the effluent holding tank.

In a preferred embodiment, an acidic solution is contained in the neutralization solution tank and is used to neutralize the waste products in the pH treatment tank. The acidic solution can comprise hydrochloric acid. The acidic solution can further comprise a chlorite neutralizing compound. In a further embodiment, the chlorite neutralizing compound is selected from the group of ferrous chloride tetra hydrate, sodium sulfite, sodium metabisulfite, and sodium thiosulfate pentahydrate.

A method for neutralizing waste products from an electrochemical chlorine dioxide generator comprises collecting waste products from the electrochemical generator in a pH treatment tank. A neutralizing solution is added to the waste product in the pH treatment tank. The waste products are recirculated to achieve at least one of the removal of substantially all chlorite and a pH between 4 and 10. The waste products are transferred from the pH treatment tank for storage and/or disposal. A preferred embodiment further comprises controlling the neutralizing of waste products with a PLC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of a ClO₂ generator of the type described in application Ser. No. 10/902,681 that includes a neutralization system for waste products from the electrochemical generator.

FIG. 2 is a process flow diagram of an anolyte loop of a ClO₂ generator of the type described in the '681 application.

FIG. 3 is a process flow diagram of a catholyte loop of a ClO₂ generator of the type described in the '681 application.

FIG. 4 is a process flow diagram of an absorption loop of a ClO₂ generator of the type described in the '681 application.

FIG. 5 is a process flow diagram of a neutralization system for waste products from an electrochemical ClO₂ generator.

These and other features of the apparatus and method disclosed herein will become more readily apparent to those having ordinary skill in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

FIG. 1 illustrates a process flow diagram of an embodiment of a chlorine dioxide solution generator 100 of the type described in application Ser. No. 10/902,681. The process flow of FIG. 1 can consist of three sub-processes including an anolyte loop 102, a catholyte loop 104 and an absorption loop 106. The purpose of anolyte loop 102 is to produce a ClO₂ gas by oxidation of chlorite, and the process can, along with the catholyte loop process 104, be referred to as a ClO₂ gas generator loop. The ClO₂ gas generator loop is essentially a ClO₂ gas source. Various sources of ClO₂ are available and known in the water treatment field. Catholyte loop 104 of the ClO₂ gas generator loop produces sodium hydroxide and hydrogen gas by reduction of water. Once the ClO₂ gas is produced in the ClO₂ gas generator loop, the ClO₂ gas can be transferred to an absorption loop 106 where the gas can be further conditioned for water treatment end-uses. The anolyte loop 102 and catholyte loop 104 can be fluidly connected to a neutralization system 500 that can process wastes produced by the anolyte loop 102 and catholyte loop 104. The process can be operated at least partially through a PLC-based system that can include visual and/or audible displays.

In this application, the term “absorb” refers to the process of dissolving or infusing a gaseous constituent into a liquid, optionally using pressure to effect the dissolution or infusion. Here, ClO₂ gas, which is produced in the ClO₂ gas generator loop, is “absorbed” (that is, dissolved or infused) into an aqueous liquid stream directed through absorption loop 106. The neutralization system 500 can process wastes for electrochemical ClO₂ generators that produce ClO₂ gas (for example, without an absorption loop) or ClO₂ solution (for example, with an adsorption loop).

FIG. 2 illustrates an anolyte loop 102 in an embodiment of chlorine dioxide solution generator 100 of the type described in the '681 application. The contribution of anolyte loop 102 to the ClO₂ solution generator is to produce a ClO₂ gas that is directed to absorption loop 106 for further processing. The anolyte loop embodiment of FIG. 2 is for a ClO₂ gas produced using a reactant feedstock 202. In a preferred embodiment, a 25 percent by weight sodium chlorite (NaClO₂) solution can be used as reactant feedstock 202. However, feedstock concentrations ranging from 0 percent to a maximum solubility (40 percent at 17° C. in the embodiment involving NaClO₂), or other suitable method of injecting suitable electrolytes, can be employed.

The reactant feedstock 202 can be connected to a chemical metering pump 204, which delivers the reactant feedstock 202 to a recirculating connection 206 in the anolyte loop. Recirculating connection 206 in anolyte loop connects a stripper column 208 to an electrochemical cell 210. The delivery of the reactant feedstock 202 can be controlled using PLC system 108. PLC system 108 can be used to activate chemical metering pump 204 according to signals received from a pH sensor 212. pH sensor 212 is generally located along recirculating connection 206. A pH set point can be established in PLC system 108, and once the set point is reached, the delivery of reactant feedstock 202 can either start or stop.

Reactant feedstock 202 can be delivered to a positive end 214 of electrochemical cell 210 where the reactant feedstock is oxidized to form a ClO₂ gas, which is then dissolved in an electrolyte solution along with other side products. The ClO₂ solution with the side products is directed away from electrochemical cell 210 to the top of stripper column 208 where a pure ClO₂ is stripped off in a gaseous form from the other side products. Side products or byproducts can include chlorine, chlorates, chlorites and/or oxygen. The pure ClO₂ gas is then removed from stripper column 208 under a vacuum induced by gas transfer pump 216, or analogous gas or fluid transfer device (such as, for example, a vacuum-based device), where it is delivered to an adsorption loop. The remaining solution is collected at the base of stripper column 208 and recirculated back across the pH sensor 212 where additional reactant feedstock 202 can be added. The process with the reactant feedstock and/or recirculation solution being delivered into positive end 214 of electrochemical cell 210 is then repeated.

As described in the '681 application, modifications to the anolyte loop process can be made that achieve similar results. As an example, an anolyte hold tank can be used in place of a stripper column. In such a case, an inert gas or air can be blown over the surface or through the solution to separate the ClO₂ gas from the anolyte. As another example, chlorate can be reduced to produce ClO₂ in a cathode loop instead of chlorite. The ClO₂ gas would then similarly be transferred to the absorption loop. In a further example, ClO₂ can be generated by purely chemical generators and transferred to an absorption loop for further processing.

FIG. 3 illustrates a catholyte loop 104 in an embodiment of a chlorine dioxide solution generator 100 of the type described in the '681 application. Catholyte loop 104 contributes to the chlorine dioxide solution generator 100 by handling byproducts produced from the electrochemical reaction of reactant feedstock 202 solution in anolyte loop 102. As an example, where a sodium chlorite (NaClO₂) solution is used as reactant feedstock 202, sodium ions from the anolyte loop 102 migrate to catholyte loop through a cationic membrane 302, in electrochemical cell 210, to maintain charge neutrality. Water in the catholyte is reduced to produce hydroxide and hydrogen (H₂) gas. The resulting byproducts in catholyte loop, in the example of a NaClO₂ reactant feedstock, are sodium hydroxide (NaOH) and hydrogen gas. The byproducts are directed to a byproduct tank 304.

In an embodiment of catholyte loop in the example of a NaClO₂ reactant feedstock, a soft (that is, demineralized) water source 306 can be used to dilute the byproduct NaOH using a solenoid valve 308 connected between soft water source 306 and the byproduct tank 304. Solenoid valve 308 can be controlled with PLC system 108. In a preferred embodiment, PLC system 108 can use a timing routine that maintains the NaOH concentration in a range of 5 percent to 20 percent. When byproduct tank 304 reaches a predetermined level above the base of byproduct tank 304, the diluted NaOH byproduct above that level is removed from catholyte loop.

In the example of a NaClO₂ reactant feedstock, the catholyte loop self-circulates using the lifting properties of the H₂ byproduct gas formed during the electrochemical process and forced water feed from soft water source 306. The H₂ gas rises up in byproduct tank 304 where there is a hydrogen disengager 310. The H₂ gas can be diluted with air in hydrogen disengager 310 to a concentration of less than 0.5 percent. The diluted H₂ gas can be discharged from catholyte loop 104 and chlorine dioxide solution generator 100 using a blower 312.

As described in the '681 application, in another embodiment, dilute sodium hydroxide can be fed instead of water to produce concentrated sodium hydroxide. Oxygen or air can also be used as a reductant instead of water to reduce overall operation voltage since oxygen reduces at lower voltage than water.

The reaction of anolyte loop 102 and catholyte loop 104 in the embodiment illustrated in FIGS. 2 and 3 is represented by the following net chemical equation: 2NaClO_(2(aq))+2H₂O→2ClO_(2(gas))+2NaOH_((aq))+H_(2(gas)) The NaClO₂ is provided by reactant feedstock 202 of anolyte loop 102. The NaOH and H₂ gas are byproducts of the reaction in catholyte loop 104. The ClO₂ solution along with the starting unreacted NaClO₂ and other side products are directed to the stripper column for separating into ClO₂ gas as part of anolyte loop 102 process. Chlorite salts other than NaClO₂ can be used in anolyte loop 102.

FIG. 4 illustrates an absorption loop 106 of an embodiment of a chlorine dioxide solution generator 100 of the type described in the '681 application. The absorption loop processes the ClO₂ gas from anolyte loop 102 into a ClO₂ solution that is ready to be directed to the water selected for treatment.

ClO₂ gas is removed from stripper column 208 of anolyte loop 102 using gas transfer pump 216. In a preferred embodiment, a gas transfer pump 216 can be used that is “V” rated at 75 Torr (10 kPa) with a discharge rate of 34 liters per minute. The vacuum and delivery rate of gas transfer pump 216 can vary depending upon the free space in stripper column 208 and desired delivery rate of ClO₂ solution.

The ClO₂ gas removed from stripper column 208 using gas transfer pump 216 is directed to an absorber tank 402 of the absorption loop. In a preferred embodiment, discharge side 404 of gas transfer pump 216 delivers ClO₂ gas into a 0.5-inch (13-mm) poly(vinyl chloride) (PVC) injection line 406 external to absorber tank 402. Injection line 406 is an external bypass for fluid between the lower to the upper portions of the absorber tank 402. A gas injection line can be connected to injection line 406 using a T-connection 408. Before ClO₂ gas is directed to absorber tank 402, the tank 402 is filled with water to approximately 0.5 inch (13 mm) below a main level control 410. Main level control 410 can be located below where injection line 406 connects to the upper portion of absorber tank 402. Introducing ClO₂ gas into injection line 406 can cause a liquid lift that pushes newly absorbed ClO₂ solution up past a forward-only flow switch 412 and into absorber tank 402. Flow switch 412 controls the amount of liquid delivered to absorber tank 402. Absorber tank 402 has a main control level 410 to maintain a proper tank level. In addition to main control level 410, safety control levels can be employed to maintain a high level 414 and low level 416 of liquid where main control level 410 fails. A process delivery pump 418 feeds ClO₂ solution from absorber tank 402 to the end process without including air or other gases. Process delivery pump 418 is sized to deliver a desired amount of water per minute. The amount of ClO₂ gas delivered to absorber tank 402 is set by the vacuum and delivery rate set by gas transfer pump 216.

PLC system 108 can provide a visual interface for the operator to operate the chlorine dioxide solution generator 100. PLC system 108 can automatically control the continuous operation and safety of the production of ClO₂ solution. PLC system 108 can set flow rates for anolyte loop 102 and catholyte loop 104. The safety levels of absorber tank 402 can also be enforced by PLC system 108. PLC system 108 can also control the power for achieving a desired current in an embodiment using an electrochemical cell 210. In a preferred embodiment, the current ranges from 0 to 100 amperes, although currents higher than this average are possible. The amount of current determines the amount of ClO₂ gas that is produced in anolyte loop 102. The current of the power supply can be determined by the amount of ClO₂ that is to be produced. PLC system 108 can also be used to monitor the voltage of electrochemical cell 210. In a preferred embodiment, electrochemical cell 210 can be shut down when the voltage exceeds a safe voltage level. In another preferred embodiment, 5 volts can be considered a safe voltage level.

As described in the '681 application, another operation that can be monitored with PLC system 108 is the temperature of electrochemical cell 210. If overheating occurs, PLC system 108 shuts down electrochemical cell 210. PLC system 108 can also monitor the pH of the anolyte using a pH sensor 212 (shown in FIG. 2). During operation of electrochemical cell 210, the pH of the solution circulating in anolyte loop decreases as hydrogen ions are generated. In the exemplary embodiment of the NaClO₂ reactant feedstock, when the pH goes below 5, additional reactant feedstock is added using PLC system 108. Control of pH can also be handled by adding a reactant that decreases the pH when the pH is too high.

In another embodiment, the transfer line from gas transfer pump 216 can be connected to absorber tank 402 directly without injection line 406, and can allow for increasing the pump transfer rate. Other embodiments can include a different method of monitoring the liquid level in absorber tank 402. For example, an oxidation and reduction potential (ORP) can be dipped in absorber tank 402. ORP can be used to monitor the concentration of ClO₂ in the solution in absorber tank 402. PLC system 108 can be used to set a concentration level for the ClO₂ as monitored by ORP, which provides an equivalent method of controlling the liquid level in absorber tank 402. Optical techniques such as photometers can also be used to control the liquid level in absorber tank 402. Absorption loop 106 can be a part of the chlorine dioxide solution generator or it can be installed as a separate unit outside of the chlorine dioxide solution generator. In another embodiment, process water can be fed directly in absorber tank 402 and treated water can be removed from the absorber tank 402. The process water can include a demineralized, or soft, water source 420 and the process water feed can be controlled using a solenoid valve 422.

FIG. 5 is a process flow diagram of an embodiment of a neutralization system 500 for waste products from an electrochemical ClO₂ generator 100, such as the generator described in the '681 application. As illustrated in FIG. 1, wastes from the anolyte loop 102 can be fluidly connected to the neutralization system 500 by way of a connection from the stripper column. Another embodiment illustrates the wastes from the catholyte loop 104 fluidly connected to the neutralization system 500 by way of a connection from a byproduct tank. Hydrogen byproduct wastes from the catholyte loop typically exit the byproduct tank by way of the blower 312 as shown in FIG. 3, where the hydrogen can be diluted with air and discharged into the atmosphere at a preferred concentration of less than 0.1 percent hydrogen. Caustic soda or sodium hydroxide from the catholyte loop is also typically collected in the byproduct tank 304, such as, for example, where a chlorite reactant feedstock is used in the anolyte loop 102.

FIG. 5 illustrates anolyte waste and catholyte waste directed into a pH treatment tank 510. In a preferred embodiment, both waste streams are combined into the same tank. The pH treatment tank 510 is fluidly connected to a number of pumps including a recirculation and transfer pump 520 and a neutralization feed pump 530. A single pump can perform one or more functions. For example, the recirculation and transfer pump 520 can perform both recirculation and transfer operations or two separate pumps can be used to individually perform each separate operation. The neutralization feed pump 530 can allow uninterrupted flow to the pH treatment tank 510 using a pH control system 540 that can continuously modulate the neutralization feed pump 530. The recirculation and transfer pump 520 can serve a dual function of treating (that is, mixing or neutralizing) waste products in the pH treatment tank 510 and transferring or discharging the treated waste products to an effluent holding tank 550, which can have a drain 555 to remove waste products. The effluent holding tank can also accept dilution water.

A waste product that can result in the anolyte loop 102 process is unseparated ClO₂ from the stripper column operation. In one embodiment, a typical concentration range of a chlorine dioxide waste product from the anolyte loop is 700 ppm to 1500 ppm. For a ClO₂ concentration at this level, it is difficult to tolerate the odor and the overall effects can be toxic, thus making neutralization of the toxic effects of the chlorine dioxide waste a desired outcome. Since the waste from the catholyte loop of an electrochemical chlorine dioxide generator includes sodium hydroxide, the catholyte waste can neutralize the chlorine dioxide waste produced from the anolyte loop, as represented by the following chemical equation: 2ClO₂+2NaOH→NaClO₂+NaClO₃+H₂O

The quantity of chlorine dioxide waste that is produced in the electrochemical reaction is typically much less that the quantity of sodium hydroxide. As the anolyte and catholyte waste streams enter the pH treatment tank 510 and combine, the overall mixture is typically alkaline in nature where the pH can range from approximately 12 to 13. This high pH range is due to the larger quantity of catholyte waste compared to the anolyte waste.

In consideration of various disposal options, the waste products can be neutralized with acid prior to disposal. In a preferred embodiment, hydrochloric acid or muriatic acid, such as that which is commercially available for use in swimming pools, can be used as a neutralizing agent. By way of the neutralization solution tank 520, hydrochloric acid can be added to the pH treatment tank 510. The addition of the hydrochloric acid to the combined anolyte and catholyte wastes in the pH treatment tank is preferably done with sufficient mixing to limit the formation of chlorine dioxide, which can occur when the pH in the treatment tank 510 decreases to less than approximately 4. In a preferred embodiment, the neutralization system 500 illustrated in FIG. 5 can have the pH control system 540 monitor the pH so that the addition of acid is stopped when the pH of the treated waste stream drops below 9. In another embodiment, the pH level of the treated waste stream is maintained above 4.

In addition to the alkalinity of the wastes from an electrochemical chlorine dioxide generator, the levels of chlorite from the anolyte loop wastes can also preferably be converted or neutralized. This can be done by converting the chlorite into chloride by reacting the sodium chlorite waste product with any of a number of compounds. For example, sodium chlorite can be reacted with ferrous chloride as demonstrated by the following chemical equation: NaClO₂+4FeCl₂+2H₂O→4Fe(OH)₂+NaCl In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 795.24 grams of ferrous chloride tetra hydrate.

As another example, sodium chlorite can be reacted with sodium sulfite as demonstrated by the following equation: NaClO₂+2Na₂SO₃→2Na₂SO₄+NaCl In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 252.08 grams of sodium sulfite.

In another embodiment, sodium chlorite can be reacted with sodium metabisulfite as demonstrated by the following equation: NaClO₂+Na₂S₂O₅+H₂O→2Na₂SO₄+NaHSO₄+HCl In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 190 grams of sodium metabisulfite.

Sodium chlorite can also be reacted with sodium thiosulfite as demonstrated by the following equation: NaClO₂+4HCl+4Na₂S₂O₃→2Na₂S₄O₆+2H₂O+5NaCl In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 992.68 grams of sodium thiosulfate pentahydrate.

Table 1 summarizes preferred chlorite neutralizing compounds and the concentration of neutralizer compound that will neutralize 1 ppm of chlorite into chloride. TABLE 1 Chemical concentration that will neutralize 1 ppm of chlorite Neutralizer Concentration Compound (ppm) FeCl₂.H₂O 11.8 Na₂SO₃ 3.7 Na₂S₂O₅ 2.8 Na₂(S₂O₃).5H₂O 14.7

In a preferred embodiment, the chlorite neutralizing compound, such as those described in Table 1, can be combined with a dilute acid (for example, a solution of 5 percent acid) for subsequent neutralizing of the alkaline anolyte and catholyte waste mixture held in the pH treatment tank 510. The chlorite neutralizing compound and dilute acid can be combined in the neutralization solution tank 560. The amount of neutralizer compound can be calculated based on the amount of chlorite present in the waste stream. A typical concentration range of chlorite is 0 to 0.3 moles/liter (that is, molarity (M)) in an electrochemical chlorine dioxide generator.

FIG. 5 illustrates the electrochemical chlorine dioxide generator 100 fluidly connected to the pH treatment tank 510 by way of connection to the anolyte and catholyte waste discharges from the electrochemical chlorine dioxide generator 100. Following the mixing of the catholyte and anolyte waste streams in the pH treatment tank 510, the pH of the combined wastes typically ranges from around 12 to 14. A recirculation or mixing process can be performed with the pH treatment tank 510 using the recirculation and transfer pump 520. Recirculation is typically performed when a diverting valve 570 between the recirculation and transfer pump 520 and the effluent holding tank 550 is closed. A recirculation process is preferred to help keep the concentration of the waste products uniform and diluted so that the wastes in the pH treatment tank 510 do not become acidic and cause ClO₂ to form.

In a preferred embodiment, residual chlorine dioxide from the anolyte waste stream reacts with sodium hydroxide from the catholyte waste stream to form chlorite and chlorate when the pH in the pH treatment tank 510 is maintained between approximately 12 and 14. Following the reaction of the chlorine dioxide waste, the waste product in the pH treatment tank 510 can be adjusted to have a pH of less than 10 using a dilute hydrochloric acid (HCl concentration less than 30 percent). Adjusting and maintaining a pH of less than 10 can be accomplished by monitoring the pH of the waste product in the pH treatment tank 510 using a probe 545 connected to the pH control system 540 and automatically adding the neutralizing hydrochloric acid with the neutralizing feed pump 530 which is also connected to the pH control system 540. Although the probe can be placed anywhere in the neutralization system 500 where it can monitor the pH of the product in pH treatment tank 510, in a preferred embodiment the pH probe 545 is placed in the recirculation loop. The recirculation loop includes the recirculation and transfer pump 520 and the pH treatment tank 510. The waste products in the pH treatment tank 510 are recirculated in the recirculation loop using the pump 520 to mix the waste products.

In a further embodiment, chlorite neutralizing chemicals such as those identified in Table 1 can be mixed with the dilute hydrochloric acid in the neutralization solution tank 560. Preferably, the chlorite neutralizing chemicals are in excess of the concentrations identified in Table 1. As the neutralization feed pump 530 adds neutralization solution to the pH treatment tank 510, the remaining chlorite in the wastes in the pH treatment tank 510 will be converted into chloride. In a preferred embodiment, the neutralization solution tank 560 can have double wall jacketing to provide leak containment.

In a preferred embodiment, level sensors are used to control the level of the waste products in the pH treatment tank 510. A level sensor arrangement can include a level sensor high (LSH) 580 and a level sensor low (LSL) 585. LSH 580 can open diverting valve 570 when the level in the pH treatment tank 510 is high and needs to be lowered by diverting some of the waste product to the effluent holding tank 550. When the level in the pH treatment tank decreases to a level that triggers LSL 585, the diverting valve 550 will close. Any additional diversion of waste product to the effluent holding tank 550 occurs by opening the diverting valve 570. The diverting valve can also be controlled by a pH sensor 547 that limits waste products of high or low pH from being discharged into the effluent holding tank 550.

In a preferred embodiment, the neutralization system 500 and/or electrochemical chlorine dioxide generator 100 can be controlled with a PLC 590. For example, for the neutralization system 500, the PLC 590 can be used to control the neutralization feed pump 530, the recirculation and transfer pump 520, the level sensors 580, 585, the diverting valve 570, the pH probe 545, pH sensor 547, and/or the pH control system 540. The PLC 590 can be further used to control the electrochemical chlorine dioxide generator 100.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings. 

1. An electrochemical chlorine dioxide generator comprising: (a) an anolyte loop fluidly connected to a neutralization system; and (b) a catholyte loop fluidly connected said neutralization system; wherein said neutralization system neutralizes waste products from at least one of said anolyte loop and said catholyte loop.
 2. The electrochemical chlorine dioxide generator of claim 1, wherein an absorption loop is fluidly connected to said anolyte loop.
 3. The electrochemical chlorine dioxide generator of claim 1, wherein a stripper column within said anolyte loop is fluidly connected to said neutralization system.
 4. The electrochemical chlorine dioxide generator of claim 1, further comprising a pH control system to monitor the pH of said waste products in said neutralization system.
 5. The electrochemical chlorine dioxide generator of claim 1, wherein at least one of said electrochemical chlorine dioxide generator and neutralization system is controlled with a programmable logic controller.
 6. The electrochemical chlorine dioxide generator of claim 1, wherein said neutralization system comprises a pH treatment tank for receiving waste products, wherein said pH treatment tank is fluidly connected to at least one of said anolyte loop and said catholyte loop.
 7. The electrochemical chlorine dioxide generator of claim 6, wherein said neutralization system further comprises a neutralization solution tank fluidly connected to said pH treatment tank.
 8. The electrochemical chlorine dioxide generator of claim 6, further comprising a recirculation pump fluidly connected to said pH treatment tank, wherein said recirculation pump mixes said waste products in said pH treatment tank.
 9. The electrochemical chlorine dioxide generator of claim 6, further comprising an effluent holding tank fluidly connected to said pH treatment tank, wherein said effluent holding tank receives neutralized waste products.
 10. The electrochemical chlorine dioxide generator of claim 9, further comprising a transfer pump for transferring said neutralized waste products to said effluent holding tank, wherein a diverting valve controls said fluid connection between said pH treatment tank and said effluent holding tank.
 11. The electrochemical chlorine dioxide generator of claim 7, wherein an acidic solution contained in said neutralization solution tank is used to neutralize said waste products in said pH treatment tank.
 12. The electrochemical chlorine dioxide solution generator of claim 11, wherein said acidic solution comprises hydrochloric acid.
 13. The electrochemical chlorine dioxide generator of claim 11, wherein said acidic solution further comprises a chlorite neutralizing compound.
 14. The electrochemical chlorine dioxide generator of claim 13, wherein said chlorite neutralizing compound is selected from the group of ferrous chloride tetra hydrate, sodium sulfite, sodium metabisulfite, and sodium thiosulfate pentahydrate.
 15. An electrochemical chlorine dioxide generator with a neutralization system comprising: (a) a pH treatment tank for receiving waste products from said electrochemical generator; (b) a neutralization solution tank fluidly connected to said pH treatment tank, wherein a neutralization feed pump transfers a neutralization solution to said pH treatment tank; (c) a recirculation pump fluidly connected to said pH treatment tank, wherein said recirculation pump mixes said waste products in said pH treatment tank; (d) an effluent holding tank fluidly connected to said pH treatment tank; and (e) a transfer pump for transferring neutralized waste products from said pH treatment tank to said effluent holding tank.
 16. The electrochemical chlorine dioxide generator of claim 15, wherein said neutralization solution is an acidic solution.
 17. The electrochemical chlorine dioxide generator of claim 18, wherein said acidic solution further comprises a chlorite neutralizing compound.
 18. The electrochemical chlorine dioxide generator of claim 13, wherein said chlorite neutralizing compound is selected from the group of ferrous chloride tetra hydrate, sodium sulfite, sodium metabisulfite, and sodium thiosulfate pentahydrate.
 19. A method for neutralizing waste products from an electrochemical chlorine dioxide generator comprising: (a) collecting waste products from said electrochemical generator in a pH treatment tank; (b) adding a neutralizing solution to said waste product in said pH treatment tank; (c) recirculating said waste products to achieve at least one of the removal of substantially all chlorite and a pH between 4 and 10; and (d) transferring said waste products from said pH treatment tank to an effluent holding tank.
 20. The method of claim 19, further comprising controlling said neutralizing of waste products with a programmable logic controller. 